1. Field of the Invention
This invention relates to the endless press fabrics used in the press sections of papermaking machines to support, carry, and dewater the wet fibrous sheet as it is being processed into paper. It more specifically relates to seamed, rather than endless, fabrics, and to a method by which its two ends are joined and the seam formed when it is installed on the machine.
2. Background Information
Endless "belts" or fabrics are key components of all three sections (forming, pressing, and drying) of the machines used to manufacture paper products. In a sense, one of their functions can be viewed as comparable to that served by a conveyor belt, because, aside from serving the other key purposes of support and dewatering , they carry the wet fibrous sheet along as it is being converted into a paper product.
In the forming section, the forming fabric is much like a fine mesh screen. The furnish, an aqueous suspension of wood fibers and additives, is deposited onto the forming fabric in this first stage and water begins to drain through its fine mesh, leaving a wet fibrous sheet behind. The sheet is then carried to the next stage, the press section. There, the fabric belt, known in the industry as a press fabric, accepts water pressed from the sheet as it is carried through a series of presses, whose operation, in principle, is to mechanically squeeze water from the sheet into the press fabric. The press fabric, because it has a smooth surface, also imparts such a surface onto the sheet.
Finally, in the dryer section, the drying fabric belts conduct the sheet around and hold it tightly against a series of steam-heated rolls to evaporate the remaining water. For this reason, these fabric belts are usually lighter and thinner and more permeable than those used in the press section because they must allow water vapor to freely pass from the sheet.
Generally, the fabrics used in the press section are supplied in endless form, that is, a fabric woven in the form of an endless loop without a seam. On the other hand, drying fabrics are typically provided in open-ended form and require seaming on the machine when installed. The comparative ease with which an on-machine-seamed (OMS) press fabric can be installed will be our primary concern here. To carry out such an installation, one merely has to draw one end of the open-ended fabric through the machine and around the appropriate guide and tension rolls and other components. Then, the two ends can be joined at a convenient location on the machine and the tension adjusted to make the fabric taut. In fact, a new fabric can be installed at the time an old one is removed. In this case, one end of the new fabric is connected to the old fabric, which is used to pull the new fabric into proper position on the machine.
By way of contrast, the installation of an endless fabric in a press section is a much more difficult and time-fabric consuming undertaking. The machine must, of course, be shut down and the old fabric cut out or otherwise removed. The new fabric then must be slipped into the machine from the side into the spaces between the presses and around other machine components and through spaces in the machine frame. The difficulty is compounded by the fact that the newer generation press fabrics have been becoming increasingly bulkier and stiffer. This increases the time and effort required by plant personnel to install a new one. Viewed in this light, the development of an OMS press fabric is highly desirable.
There are a number of different ways by which a seam might be formed and closed, and which have been used on drying fabrics. One of these ways is the clipper seam, which consists of closely spaced metal hooks at each end of the belt. The seam is formed by clipping the hooks from both ends onto a common metal cable. The resulting seam is a good deal thicker than the body of the fabric and carries with it the disadvantage that the metal hooks eventually corrode and weaken.
Other, non-metallic, seams include the spiral, multifilament, and pin seam. Generally, each involves the joining of the two ends of the belt by running a non-metallic cable through similar loops at each end of the fabric. More specifically, the ends of the fabric are brought closely together in such a way that the loops on one end of the fabric alternate and mesh with those of the other end. Then, the seam is closed by passing a cable or strand through the meshed loops joining the two ends together.
Except for the pin seam, these share one major disadvantage of the clipper seam by being thicker than the rest of the body of the fabric. This is because they involve the addition of other elements onto the ends of the fabric which are then used to close the seam. In addition to increasing the thickness of the fabric at the point of the seam, these added elements are subject themselves to wear and failure and their presence changes the porosity characteristics of the fabric in the region of the seam.
The pin seam, however, is more difficult to distinguish from the rest of the fabric. To close this seam again requires that a cable or strand, the pin, be passed through the meshed loops of each end of the fabric. This can be accomplished in two ways. In the first case the loops are formed by the machine direction strands themselves, looped and woven back into the fabric. The second technique employs the art of weaving "endless", which normally results in a continuous loop of fabric. However, when making a pin-seamable press fabric, one edge of the fabric is woven in such a way that body yarns form loops, one set of alternating loops for each end of the woven cloth. Using these techniques, the seam location will be more nearly the same thickness as the rest of the fabric.
The type of seam used in a drying fabric has not been a matter of major concern. In the drying section, as mentioned above, the fabric conducts the sheet around a series of heated rolls. Tension in the drying fabric is relatively low when compared to that on a press fabric. The drying fabric is not subject to any compression which might otherwise damage the seam. Finally, the risk of sheet-marking by the seam is small at this point, because much of the paper sheet water has been removed.
The thickness of the seam would be a major concern, however, in the press section. If the seam were considerably thicker than the body of the fabric, it would be subject to high compressive forces on each passage through a press. Not only would this lead to damage to the seam and a shortened seam life, but also press roll vibrations could be set up by the repetitive effect of the thick seam passing through. In addition, the wet fibrous sheet, still quite fragile in the press section, would be marked, if not partially or wholly broken, by the higher compressive force at the seam location.
A pin seam, however, can be used in attempting to develop an OMS press fabric without a thickness-related problem. The first open-ended press fabrics using pin seams were closed with pintles, as the pins passing through the loops are called. Used for the same purpose in drying fabrics, they were the same size, solid, round shape, and of the same material. However, as tensions in a press fabric are four to ten times higher than those in a drying fabric, the same pintles were not up to the task of holding the seams intact.
In addition, sheet marking was observed in spite of the use of the pin seam. Investigation disclosed that this was sometimes being caused by the presence of a small gap in the fabric at the location of the seam. This occurred because the pintle did not completely fill the space formed by the loops. As a result, sheet water was not being properly removed from the region of the sheet adjacent to the seam, causing sheet marking. The use of a single large pintle also caused the seam to be considerably thicker under compressive load than the body of the fabric. Two or more slightly smaller diameter pintles were then tried, with some improvement, but not elimination of the problem.
Use of seamed press fabrics also resulted in a new problem. Since the seam is physically and geometrically different from the body of the fabric, liquid flow in the seam area is different. This flow resistance variation allowed instantaneous larger amounts of air to pass through the seam area as the fabric passed over a mechanical dewatering device called a suction box. This caused a loud "popping" noise to occur, causing great concern to the papermachine operating people. Thus, another requirement for the pintle was added.
One attempt to overcome this difficulty consisted of the use of a stuffer yarn with the pintle in order to completely fill the space formed by the loops. This approach, however, was time-consuming and not very effective in preventing the sheet mark or the loud "popping" noise produced by vacuum or air flow surge.
The primary requirements for a successful pintle are a high tensile strength both lengthwise and radially, an ability to withstand radial and shear forces from the loops, and resistance to wear from constant contact with the loops as the fabric flexes and bends around rolls in the fabric run. It must also aid the seam area in resisting sheet mark and the loud "popping" noise as the seam passes over the box. In addition, some degree of porosity is desirable so that the pintle, even when completely filling the space formed by the loops, will have a permeability more like that of the body of the fabric.
Attempts to deal with this problem through the use of single, solid larger diameter round pintles or a pair, or more, of smaller diameter round pintles yielded slight improvement. This is partly because the void to be occupied by the pintle does not have a circular cross-section. Instead, when a load is applied, this void formed by the loops becomes oblate. Therefore, the ideal pintle would have a similarly shaped cross-section and would, in consequence, contribute to a better seam with a lower profile and smaller gaps.
A monofilament, extruded in such a way that its cross section be oblate, would seem to be an appropriate pintle. However, while completing a seam with little or no gap, it would still be subject to the shortcoming of lack of permeability. It is likely then that the sheet will still be marked because of insufficient drainage of water.
Another factor to consider is that the technology of seamed press fabrics has improved in that different weaves, machine-direction (MD) loop yarn sizes and counts have been developed. The seam area therefore is different for different fabrics. Also, these different fabrics will compact somewhat in the base over the running life of the fabric. The degree of compaction depends upon the particular position on which the fabric is installed and its environment and is not predictable. An ideal pintle system therefore is one that will conform to these changes throughout the life of the fabric.
A solution to these problems is represented by a pintle whose structure conforms to the oblate seam area under load and is porous and permeable to water and air, such that the seam area acts like the main body of the fabric as far as the paper sheet is concerned. The present invention provides a pintle satisfying these criteria.